72 AN INTRODUCTION TO THE STUDY OF VIRUSES 
(because they generate the antibodies) and a substance which can act 
as an antigen is said to be antigenic (Boyd, 1946). 
An antigen must have two properties: first it must have the power 
to stimulate the production of the antibody and secondly it must 
react specifically with that antibody. 
There are four main types of this reaction between antigen and 
antibody— 
(1) Neutralization of the properties of the virus; 
(2) Complement fixation test: When antigens are mixed with their 
specific antibodies the mixture has the property of removing the 
power of normal serum to haemolyse sensitized red corpusles. Com- 
plement are a heat labile substance present in normal blood serum 
and the reaction is a kind of delicate colour indicator test; 
(3) Precipitin reaction: A precipitate is formed when the virus is 
added to its specific antiserum in saline at different dilutions and 
warmed in a water-bath. In precipitation the antibody is referred to as 
precipitin; 
(4) Anaphylaxis: See page 77. 
The antiserum (or antibodies) to a given virus is produced by in- 
jecting a susceptible animal with a suspension of the virus in question. 
Rabbits can be used for some viruses and are easily immunized against 
vaccinia virus. The animal is inoculated sub-cutaneously with 0-25 c.c. 
of a 1: 1000 dilution of vaccinia virus, because vaccinia is a skin 
parasite. 
Similarly, antiserum to influenza virus can be produced in rabbits: 
by the inoculation of two doses at weekly intervals of 5 c.c. of allantoic 
fluid from a culture of the virus in the developing hen’s egg. 
Guinea-pigs have also been used in the preparation of antisera to 
vaccinia, herpes, and psittacosis viruses. In the case of psittacosis, a 
mouse was first inoculated with the virus and its spleen removed and 
extracted with saline to make a 10 per cent suspension of tissue. 
It must be remembered in preparing the viruses for inoculation 
into animals that many of the accompanying proteins from the 
tissues of the host are also antigenic and therefore the virus which is 
to be used for the production of antisera must be as free from ‘con- 
taminating tissue material as possible. Viruses cannot, like bacteria, 
be grown in pure culture on non-living, synthetic media, and this 
fact adds to the difficulty of preparing a pure virus suspension. It is 
easier with some animal viruses, than with others, to prepare a virus 
